In the explanation, it says オノマトペ comes from the French word “onomatopée”. However, it’s worth noting that both the French onomatopée and the Japanese オノマトペ ultimately come from the Greek word ὀνοματοποιία (onomatopoiía), meaning “name making” (ὄνομα = name, ποιεῖν = to make). I am making this post because I am offended. Pls fix.
The Japanese borrowed directly from the French, which itself comes from Latin onomatopoeïa, which itself comes from Ancient Greek. There’s nothing to be offended about, this is how languages evolve.
ギャラリー comes from English “gallery” which itself comes from French “galerie” which itself comes from Latin galeria. You probably wouldn’t say that ギャラリー was borrowed from Latin however.
Also, the reason for WK noting that the immediate source of the Japanese word was French is that it explains (and perhaps helps with remembering) the katakana spelling, which you would probably otherwise expect to be based on the English word. Knowing that its ultimate source is Greek doesn’t help in that way.
But yeah, Japanese is full of loanwords from other languages. When sources talk about the origin of loanwords, they’re generally only referring to the language it was directly borrowed from. It can be useful to know that バイト (part-time job) comes from the German “arbeit”, but it’s perhaps less useful to know, at least for the purposes of learning Japanese, that it comes from the Proto-Indo-European *h₃órbʰos, which happens to also be the source of the English word “orphan”, and also the word “robot”. It’s interesting to know, if etymology is your thing, but doesn’t really reinforce the pronunciation or meaning of バイト.